Hi!
Well, it is really easy. First you spray your model in the desired colour, In my example green, then you spray an overall white coat but not covering all the areas! Something like preshaden but then with white.
Then comes the easiest part. You need a sponge, some water and white paint. I used Vallejo. Dilute the paint with water and then dip it in your sponge. Take a white paper and put the sponge on it. You can now see how "strong" the chips are. (sorry for my bad english). If the chips are allright then you can use the sponge on your model. You can also make vertical strokes with the sponge to brake the chipped effect.
Well, it is difficult to explain but you should just give it a try. It is really dead easy!
Good luck and show us your results!
Jurjen